Messages in this thread | | | Date | Mon, 29 Apr 2013 11:58:49 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 3/3] x86, FPU: Do not use static_cpu_has before alternatives |
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On 04/29/2013 11:51 AM, Borislav Petkov wrote: > On Mon, Apr 29, 2013 at 08:42:30AM -0700, H. Peter Anvin wrote: >> I *was* considering adding static_cpu_has_safe() at some point which >> would have a three-state jump, with the default (pre-alternatives) >> jump pointing to dynamic detection code. > > Actually, if we teach __static_cpu_has to do something like > ALTERNATIVE_JUMP arch/x86/lib/copy_user_64.S but make the second > alternative insn alt2 be none, i.e. no replacement, we can have: > > * pre-alternatives: JMP dynamic_detection > * post-alternatives: > - feature present: delete JMP > - feature absent: s/dynamic_detection/t_no/, i.e., patch only the label.
That is what I was referring to, yes.
> And even though asm goto supports multiple labels, we need to be able > to either patch the label only or patch out the whole instruction - > otherwise we'll be adding additional NOP bytes.
This doesn't seem to matter.
> I wonder if it would make sense to teach the alternatives to skip the > opcode when patching so that we can say: "we only want to patch the > label so we're patching in the offset now but leaving the single JMP > opcode in there." > > But for that we either need flags in struct alt_instr or do something > ad-hoc apply_alternatives already does for relative jumps (0xe8). > >> This might be useful here, on the other hand, perhaps it is acceptable >> for use_eager_fpu() to be initially false? > > Hmm, I don't know, FPU code is crazy. OTOH, does CR0.TS even matter on > non-lazy FPU restore machines?
Yes, CR0.TS should be zero, or we'll get #NM traps when user space tries to access the FPU. For lazy restore CR0.TS should be set so we get the #NM trap and can restore the FPU.
-hpa
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